Playa could create a themed small Dungeon (like 10-30 floors) that would be available for only a week during a year. Each monster would not regenerate so with consecutive attacks you could for sure take enemies down.
And the rewards would be appropriate to monster levels, which would change quite a lot with each beaten enemy.

So let's get an example here:

We create a 20 floor dungeon with classic Robin Hood characters (depicted from movies and tv series) that would only be available at the start of June each year.
Monter levels would look something like that:
1) lvl 10 2) lvl 30 3) lvl 60 4) lvl 100 5) lvl 150 6) lvl 210 7) lvl 290 and so on.

Sure, progression would be drastically different based on server age but that's the point. And monsters would not regenerate so you would be able to progress a bit further then your level would suggest.

The beauty of this is that next year it would be available again from start and everyone would have a nice little event.

Also Playa could easily add other events in the future, creating a whole calendar of special events.
On some you could battle yourself, on some with your tower followers and maybe there would be a battle in Helm's Deep (from Lord of the Rings) where you would use your Fortress Army.